1. Field of the Invention
The field of the invention is that of the transmission of digital data, especially in transmission channels showing or being capable of showing fast Doppler fading and low inter-symbol interference (ISI). More specifically, the invention relates to the estimation of the transmission channel and the demodulation in receivers of signals transmitted through such channels.
A preferred but not exclusive field of application of the invention is that of digital communications between satellites and moving bodies. It can be envisaged especially in the context of the ICO and Iridium projects. More specifically, the invention can be applied advantageously in all communication systems where the channel has fast Doppler fading and low ISI.
2. Description of the Prior Art
In conventional digital communication systems, synchronization symbols are frequently used. They enable the receiver not only to get synchronized but also to estimate the channel suitably in order to ensure the efficient running of the demodulation phase.
For these conventional communication systems, the synchronization symbols are consecutive and form a synchronization sequence that is generally placed at the beginning of the data train. The synchronization in the receiver is achieved by detecting a crossing of a threshold of the correlation of samples received with those of the sequence used for the synchronization. This synchronization is used firstly to detect the beginning of a data train and secondly to achieve a precise definition of the instants of maximum aperture of the eye diagram.
In the case of studies carried out for the GSM system, it has been sought to build synchronization sequences matched with the slow Doppler fading and high ISI channels. These studies have shown the value, for channels of this type, of placing these synchronization sequences in the middle of each data train.
The patterns of synchronization sequences proposed lead to synchronization and channel estimation algorithms that are intuitive and simple to implement. They are therefore adopted in several digital communications systems.
They are also envisaged in satellite telephony projects. However, they prove to be far less efficient for channels of this type and lead to major limitations of performance when channels with fast Doppler fading and low ISI such as satellite channels are considered.
The basic principle of temporal synchronization and channel estimation by the receiver is that of transmitting a sequence of symbols known to this receiver in the data train sent by the transmitter. Through the use of a few predefined algorithms, these symbols are exploited to ensure not only efficient synchronization of the receiver but also reliable estimation of the channel, thus enabling the efficient running of the demodulation phase.
The known systems make use of a synchronization pattern consisting of a sequence of symbols grouped together, having efficient correlation properties. These properties are profitably used chiefly to achieve efficient synchronization at the receiver. In the context of radio-mobile communication systems such as the GSM system, the channel is almost static during a data train but nevertheless undergoes very severe levels of ISI. These correlation properties then prove to be well suited and even necessary for a simple and direct estimation of the pulse response of the channel.
These synchronization sequences may be used in communication systems between satellites and moving bodies. It must be noted however that if the channels encountered in applications of this type have negligible ISI levels in relation to the symbol rate, their variations in terms of the data train received are great. In other words, the transmission channel then cannot be considered as being quasi-static during the period of a data train (in terms of time/frequency representation it can be said that these channels are dual to those presented here above).
Consequently, the use of a standard synchronization sequence in systems of this type is purely arbitrary, and resolves only the problem of synchronization in the receiver.
More specifically, while the choice of group synchronization in the context of quasi-static channels with high ISI (the standard case of GSM in particular) has a sound basis, this is not necessarily so for channels with fast Doppler fading and low ISI. These channels have considerable variations in terms of the data trains received. The sequence of standard synchronization symbols may ensure that the receiver is well synchronized. By contrast, the quality of channel estimation is severely compromised as it does not represent the state of the channel during the reception of the useful data elements of a data train.
The standard synchronization therefore is not suited to systems communicating through a channel with fast Doppler fading, for the channel estimation and demodulation function.